This invention generally relates to thin film processing systems in which a plasma effects either etching, deposition, or some other process. Specifically, the invention has application to DC plasma processing when coating with metallic materials or with materials formed by chemical reaction in the coating process. It also involves power supply designs used in such applications.
The field of DC plasma processing for thin film applications is one which is well known. In these processes, a DC power supply creates an electric potential between a cathode and anode and thereby creates a plasma. In the deposition mode, the plasma then acts upon a material target to create a thin film on some substrate. This thin film may either be comprised of the target material itself or may be the result of some reaction with some element within the coating chamber. Naturally both the materials and elements involved and the specific applications vary greatly. Applications may range from coating architectural glass to the creation of micro chips. One of the challenges in many applications is that electrical discharges or arcs can occur. This is particularly true when reactive processes are used and the reactive product is an insulator, such as aluminum oxide (Al.sub.2 O.sub.3). As one example, this type of coating process is particularly challenging because it involves both conductive and insulating regions. As a result, the electrical environment during plasma processing itself can be particularly conducive to arc discharges. These arc discharges are undesirable not only because they represent potential non-uniformities in the coating process, but also because they can further lead to unstable situations by altering the release of coating material and negatively impact processing throughput.
Although the problem of arc occurrences has been well known to those skilled in the art, it has been addressed with only limited success. Initially it was common to completely shut down the process and perhaps even clean the chamber before restarting. In other instances, lower processing rates were used to make the occurrences of arcs less frequent. More recently, it has been attempted to divert the arc by quickly shutting off the supply of power to the plasma itself. Unfortunately, most such solutions acted only after damage had been done and thus served to minimize--but not completely avoid--problems in more sensitive processing environments. In order to react as quickly as possible, switch-mode or low energy storage power supplies have also been used for many applications. In spite of the fact that they inherently store less power and thus can be manipulated to minimize the negative effects of such arc occurrences, their use alone has not been sufficient for many processing environments. Interestingly, solutions by component designers have often been utilized without full explanation to those involved in the processing itself. This proprietary nature may have even lead to duplication of efforts and limited progress in understanding the nature of the problem. The development of solutions has primarily been the result of individual effort, not a coordinated approach. One other solution which has been publicly pursued has been the utilization of frequency-oriented components to charge a capacitor and then reverse current to negate the arc itself. Unfortunately this solution may act to increase the current in the arc initially and thus can intensify the problem before solving it. Again, this solution is undesirable especially in refined processing environments.
The present invention acts to minimize and in many instances completely eliminate the occurrence of arcs in even the most demanding processing environments. By providing solutions which have general application, the invention makes numerous solutions possible to achieve the same ends. Through its basic understandings, it thus encompasses a variety of designs and systems as they may be incorporated to solve the problem of arcing. In doing so, the present invention satisfies a long felt need for such capability. Perhaps surprisingly, the invention is based upon aspects which have been readily available to those skilled in the art, but which had not been applied in this manner. While those skilled in the art appreciated that the problem of arcing existed, they apparently did not understand the nature of the problem and therefore conducted attempts to solve the problem which were actually directed away from the direction taken by the present invention. This teaching away from the technical direction taken by the present inventors was perhaps further intensified by the fact that those skilled in the art had often avoided full disclosure of their endeavors. While certain designs may have been founded upon principles similar to those of the present invention, in many instances the lack of public disclosure has resulted in causing those skilled in the art to actually have been taught away from the direction taken by the present invention.